Japanese OPI 8-213259 describes the conventional ignition coil as used in internal combustion engines. As shown in FIGS. 11 and 12, an ignition coil having an open magnetic path comprises transformer 7 composed of primary coil 3 surrounding secondary coil 5 which, in turn, surrounds magnetic core 1. To prevent magnetic leakage, outer cylinder 9 is disposed around transformer 7. This structure is relatively compact, having a small diameter.
Plate-shaped magnetic member 11 is at one or both ends of magnetic core 1 and provides reverse bias for magnetic field B1 which, in turn, is generated by primary coil 3. The residual magnetic flux density in magnetic core 1, generated by primary coil 3, is decreased by the coercive force from magnetic member 11. When a direct current voltage is applied in bursts to primary coil 3, the changes in flux density in magnetic core 1 are increased, thus providing more efficient energy retrieval at secondary coil 5.
To supplement magnetic core 1, outer cylinder 9 is provided. However, because the magnetic path between magnetic core 1 and outer cylinder 9 is interrupted, the actual magnetic leakage is comparatively high. This impairs the use of magnetic field B1 and makes the retrieval of energy less efficient.
A large proportion of magnetic field B1, extending from the end of magnetic core 1 to the end of outer cylinder 9, is along a direction perpendicular to the axis of the magnetic core. Magnetic field A1, generated by magnet member 11, is formed along the thickness of the magnet member, i.e. axially of magnetic core 1. As a result, magnetic field B1 is not weakened by magnet member 11; on the contrary, magnetic field B1, formed between magnetic core 1 and outer cylinder 9, avoids magnet member 11. Therefore, reverse bias magnetic field A1 cannot efficiently counter magnetic field B1. This additionally prevents the secondary output from increasing. FIGS. 11 and 12 show composite magnetic field C1 formed by magnetic field B1 and magnetic field A1. As can particularly be seen in FIG. 12, composite magnetic field C1 avoids magnetic member 11 and is thus not weakened.
Japanese OPI 3-154311 discloses an ignition coil with a ring-shaped permanent magnet as the reverse-biasing magnet member. However, this patent makes no mention of the direction of the magnetic field generated by the magnet member, and the manner of application of the reverse-biasing magnetic field is unclear. If the magnet member generates a field along the thickness axis thereof, as is the case in the conventional technology shown in FIGS. 11 and 12, then a suitable reverse-biasing magnetic field cannot be achieved for the same reasons as set forth above. On the other hand, if the magnetic member generates a field in the radial direction, then the volume of the permanent magnet will be insufficient, since it must be located within the ring-shaped core. For this reason, a reverse-biasing magnetic field of adequate strength cannot be obtained in this manner.